When do units heal after a Challenge spell is played?

Units do not heal after a Challenge spell is played.

According to the FAQ regarding Challenge:

• Ruling: Units do not heal after using Challenge. They remain damaged until they heal after combat or at the end of turn.
• Nuances: Challenge is not combat, so it does not trigger the healing that occurs after a combat showdown.

As established in the game rules, units only heal at two specific times:

1. During a Combat Cleanup: This occurs after any combat showdown resolves.
2. At the end of each player's turn: This occurs during the End of Turn Cleanup step (Rule 142.3.b and Rule 317.2.b).

Damage from spells and abilities (including Challenge) persists on units until one of these two healing windows occurs.